nanolitmus paper: a wide range colorimetric ph sensor based on formation of gold@silver nanorods

Abstract: the potential of hydrogen (ph) has been defined as a significant parameter that provides crucial information on local chemical conditions. in this regard, the measurement of ph has received great attention in diverse fields such as clinical diagnostics, agriculture, food/beverage industry, and environmental/biomedical applications [1]. due to the widespread applications of ph in diverse fields, the development of rapid and simple yet reliable probes for the determination of ph has attracted significant interest. in this regard, the appearance of nanoparticles (nps) can be considered to be a milestone in the design of optical ph sensors. among the diverse types of nps, noble metal nps with unique localized surface plasmon resonance (lspr) properties, associated with their composition, size, and morphology have been considered the exciting colorimetric reporters [2]. in the present work, a multicolor colorimetric probe has been developed for ph monitoring by exploiting the silver metallization on the surface of aunrs. the reduction of silver ions by ascorbic acid is strongly influenced by ph, resulting in silver nanoshell deposition on the surface of aunrs. the formation of plasmon-tunable au@ag core-shells caused a blue shift in the longitudinal plasmonic peak of aunrs in different phs ranging from 2.0 to 12.0 at optimum conditions. as a result of spectral shifting, distinct color variations were observed from brownish pink/light green color (acidic media) to dark green/blue color (neutral media) and eventually purple/brown color (basic media). furthermore, a paper-based analytical device (pad) was introduced for on-site ph screening. the ph-sensitive pad was simply fabricated by immobilizing aunrs on the surface of filter paper. finally, the developed plasmonic probe was validated to be capable of accurate ph monitoring in urine, spoiled meat, drinking water, and seawater samples.